As is known, efficient transmission of optical wave energy along a multimode optical fiber is achieved by grading the refractive index of the fiber core. Thus, accurate knowledge of the index profile is necessary in order to assess a fiber's transmission properties. To this end, various techniques have been developed for measuring the index profile of optical fibers and optical fiber preforms. For example, U.S. Pat. No. 4,161,656 discloses various techniques for determining the distribution of the index modifying dopants in optical fibers and fiber preforms by observing the reaction of a fiber/preform to ultraviolet (uv) illumination. From this measurement, the index profile can be computed.
The shortcoming of this and various other measuring techniques is that they can only be employed after the fiber or preform has been made. A preferred arrangement would involve making measurements while the preform is being fabricated and, thereby, providing information with which to control the fabrication process itself. By this means, the resulting index distribution within the preform could be made to more closely approximate the desired profile. It is clear that such an approach would serve to reduce the number of unacceptable preforms produced and, thereby, reduce the cost of fabricating optical fibers.